1. Field of the Invention
The present invention relates to a printing apparatus and printing control method thereof, and particularly to a printing apparatus, which performs multipass printing by time-divisionally driving a plurality of printing elements of an inkjet printhead, and a printing control method thereof.
2. Description of the Related Art
Of many inkjet printing apparatuses, a serial type inkjet printing apparatus, in which a printhead including a plurality of nozzles is mounted on a carriage, and which forms a print image by repeating a carriage scan and intermittent conveyance of a print medium, has prevailed, since it is inexpensive and compact.
In such a printing apparatus, density unevenness often occurs on a print image due to variations of nozzle diameters and of ink discharge directions. In order to suppress this density unevenness, multipass printing, which completes printing by complementing one pixel by printing operations of a plurality of carriage scans, is used. Multipass printing has the aforementioned advantage, but it also has a disadvantage. That is, in the plurality of carriage scans required to complete printing, unexpected ink-landing position shifts caused by, for example, an uneven surface of a print medium, have occurred between a certain scan and another scan, thus causing density unevenness of an image on that occurrence area.
To solve such a problem, for example, Japanese Patent Laid-Open No. 2000-103088 has proposed the following method. That is, multi-valued image data is divided into data for a plurality of times used to scan a predetermined area, data conversion of the divided multi-valued image data is performed using different coefficients, and binarization processing is applied to the respective converted data. According to this method, since some pixels have an opportunity of receiving ink discharged twice or more in a plurality of print scans, a situation in which all pixels have a complementary relation can be avoided. As a result, multipass printing, which hardly causes density changes of an image even when ink-landing position shifts have occurred between print scans, can be realized.
The aforementioned related art has a sufficiently high effect when one dot per pixel is allotted on an average. However, when a high-density image is to be output, dots more than one dot per pixel have to be allotted, and in such a case a new problem is posed.
FIGS. 21A to 21C are views for explaining the conventional problem.
FIG. 21A shows a conventional dot allotment when all pixels are completely complemented without using the method proposed by Japanese Patent Laid-Open No. 2000-103088. Also, FIG. 21B shows a dot allotment when some non-complemented pixels are generated using the method proposed by Japanese Patent Laid-Open No. 2000-103088. Furthermore, FIG. 21C shows a dot allotment when two dots are allotted per pixel using Japanese Patent Laid-Open No. 2000-103088.
Black dots shown in FIGS. 21A to 21C are allotted in a first print scan, white dots are allotted in a second print scan, and gray dots are allotted in the first and second print scans. The left figure of each of FIGS. 21A to 21C shows a case free from any ink-landing position shifts between the first and second print scans, and the right figure shows a case in which ink-landing position shifts have occurred between the first and second print scans.
In the case shown in FIG. 21A, if no ink-landing position shifts occur between the print scans, white and black dots are neatly aligned in a checkerboard pattern. However, when ink-landing position shifts have occurred, white dots get closer to black dots, and partially overlap each other. Thus, a state in which a print area is not filled is formed compared to an original state. In this manner, when ink-landing position shifts have occurred at certain timings in an image area, the state shown in the left figure in FIG. 21A and that shown in the right figure are formed at adjacent positions, and this state is visually recognized as density unevenness.
On the other hand, the case shown in FIG. 21B includes pixels on which no dots are printed, and those on which dots are printed by both the first and second print scans when no ink-landing position shifts occur. Hence, when ink-landing position shifts have occurred, originally overlapped dots appear, and separated dots overlap each other, thus suppressing a density change due to the presence/absence of ink-landing position shifts as a whole. The reason why white dots and black dots have different allotments even when ink-landing position shifts do not occur is that parameters associated with binarization processing (error diffusion processing in this case) are different for white dots and black dots. In this case, when an image having a higher density than the example shown in FIG. 21B, generation ratios of both white dots and black dots have to be increased.
The case shown in FIG. 21B has higher degrees of freedom in pixel positions where both white and black dots are allotted. However, when dot generation ratios are to be increased, the number of dots to be allotted has to be increased, thus lowering degrees of freedom in allotment. This cannot be coped with by the data ratios between the first and second print scans and the contents of a diffusion matrix described in Japanese Patent Laid-Open No. 2000-103088. When dot generation ratios are to be increased, the state shown in the left figure of FIG. 21C is reached at last, and all pixels are configured by gray dots printed in the first and second print scans, thus printing an image having a highest density. However, in this state, all overlapped dots unwantedly appear when ink-landing position shifts between scans have occurred, and density changes caused by the presence/absence of ink-landing position shifts become large, as shown in the right figure of FIG. 21C. As a result, such state is visually recognized as density unevenness, as described above.